A subset of human T-cell acute lymphoblastic tumors contains a chromosomal translocation involving the beta T-cell receptor gene at chromosome 7q34 and the TAN-1 gene at chromosome 9q34. TAN-1 bears striking homology to Notch, a Drosophila gene influencing cell fate decisions in early embryogenesis. The function of TAN-1 in humans is unknown. In a survey of fetal and adult tissues, TAN-1 exhibited high expression levels in the thymus. Recently, a truncated version of the TAN-1 gene, inserted into bone marrow stem cells, has been found to induce T-cell leukemia in mice. Through a series of studies described in this application, we propose to investigate the distribution of the tan-1 protein during normal T-cell maturation, and to establish structural requirements for the transforming ability of tan-1. We will conduct immunohistochemical studies to determine the distribution and subcellular localization of the tan-1 protein at stages of human T- cell maturation and in T-lymphoblastic neoplasms. This will be combined with Western blot analysis to identify posttranslational processing of the protein, which may affect and mediate the activity of the protein. An in vitro transformation assay will be developed to test the functional importance of various structural domains of the tan-1 protein, by introducing tan-1 deletion constructs through retroviral infection of T- cell lines. Further studies will focus more specifically on a possible correlation between nuclear localization and tan-1 function, on external factors which may activate or inhibit tan-1, and on the role of tan-1 in maintaining a transformed state in T-lymphoblastic neoplasms. Knowledge of the structure-function relationships of the tan-1 protein and its role in T- cell development will be critical in establishing the mechanism by which tan-1 induces T-cell neoplasia.